#471 getImpactedCoordinates

Using ruby to calculate the blast radius of some fireworks; cassidoo’s interview question of the week (2026-07-06).

Notes

The interview question of the week (2026-07-06):

Given an n x m grid, an odd integer size, and a coordinate (row, col) representing where a firework explodes, return all grid coordinates impacted by the blast. A firework affects every cell within Math.floor(size / 2) rows and columns of the center, clipped to the grid boundaries.

Example:
> getImpactedCoordinates(5, 5, 3, 1, 1)
> [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2],[2,0],[2,1],[2,2]]

> getImpactedCoordinates(3, 3, 1, 2, 1)
> [[2,1]]

> getImpactedCoordinates(5, 5, 3, 4, 4)
> [[3,3],[3,4],[4,3],[4,4]]

Thinking about the Problem

The problem makes things easier for us because it approximates the impact area as a square from Math.floor(size / 2) rows and columns of the center.

Since we only need to return the impacted cells, we can simply:

calculate the coordinates of the impact area, remembering to trim to grid extents
generate the impact area array by iterating between the coordinates

The main gotcha to look out for is to keep everything consistent to a 0-based coordinate system.

A Solution

Here’s a first go:

def getImpactedCoordinates(grid_width, grid_height, size, center_x, center_y)
  result = []

  radius = (size / 2.0).floor
  x_min = [0, center_x - radius].max
  x_max = [grid_width - 1, center_x + radius].min
  y_min = [0, center_y - radius].max
  y_max = [grid_height - 1, center_y + radius].min

  (x_min...x_max + 1).each do |x|
    (y_min...y_max + 1).each do |y|
      result << [x, y]
    end
  end

  result
end

And it checks out with the given examples:

$ ./example.rb 5 5 3 1 1
[[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]
$ ./example.rb 3 3 1 2 1
[[2, 1]]
$ ./example.rb 5 5 3 4 4
[[3, 3], [3, 4], [4, 3], [4, 4]]

Final Code

See example.rb for the final code.

#!/usr/bin/env ruby

def getImpactedCoordinates(grid_width, grid_height, size, center_x, center_y)
  result = []

  radius = (size / 2.0).floor
  x_min = [0, center_x - radius].max
  x_max = [grid_width - 1, center_x + radius].min
  y_min = [0, center_y - radius].max
  y_max = [grid_height - 1, center_y + radius].min

  (x_min...x_max + 1).each do |x|
    (y_min...y_max + 1).each do |y|
      result << [x, y]
    end
  end

  result
end

if __FILE__==$PROGRAM_NAME
  (puts "Usage: ruby #{$0} n m size row col"; exit) unless ARGV.length == 5
  grid_width = ARGV[0].to_i
  grid_height = ARGV[1].to_i
  size = ARGV[2].to_f
  center_x = ARGV[3].to_i
  center_y = ARGV[4].to_i

  impacted_coordinates = getImpactedCoordinates(grid_width, grid_height, size, center_x, center_y)
  puts impacted_coordinates.inspect
end

Tests

I’ve setup some validation in test_example.rb:

$ ./test_example.rb
Run options: --seed 34621

# Running:

...

Finished in 0.000238s, 12605.0420 runs/s, 12605.0420 assertions/s.

3 runs, 3 assertions, 0 failures, 0 errors, 0 skips

Credits and References

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